1. Field of the Invention
The invention relates to a process for acyl substitution of an anhydride with an active-hydrogen-containing compound, more particularly to a process for acyl substitution of an anhydride with an active-hydrogen-containing compound in the presence of a vanadyl salt catalyst so as to obtain a high yield of acyl substitution reaction product with high chemoselectivity.
2. Description of the Related Art
The acylations of alcohols, amines and thiols are important and commonly used transformations in organic synthesis. In these reactions, acid halides or anhydrides are often employed as the acyl source in basic media or in the presence of Lewis base or acid catalysts. In the past, trimethylsilyl (TMS) and metal triflates, such as indium, scandium, copper, and bismuth triflates, have been found to be effective in catalyzing the acylation of alcohols with anhydrides. However, only a few of them have been studied with more extensive species of anhydrides, and substrates bearing acid-sensitive groups, such as acetonide and allyl, might not be fully compatible. Moreover, most of these catalysts have encountered some disadvantages. For example, the acylation of allyl alcohols (such as cinnamyl alcohol) catalyzed by scandium triflate would produce rearranged by-products. The acylation catalyzed by trimethylsilyl triflate should be operated at a temperature of 0xc2x0 C. or less so as to suppress the hydrolysis of the functional groups on the substrates. In addition, the actual role of the aforementioned catalysts in the catalytic pathway was not fully understood. In this application triflate refers to trifluoromethanesulfonate.
Notably, the preparation of metal triflates often require direct mixing of metal oxides with excess hot triflic acid (trifluoromethane sulfonic acid). The unavailability of complete removal of triflic acid from the metal triflates will diminish the value of the metal triflate as a catalyst because it might affect the overall catalytic activity in the acylation process. In this context, new mild and neutral catalysts, which can achieve general nucleophilic acyl substitution of anhydrides with protic nucleophiles, remain in great demand.
The value of vanadium-containing compounds as a catalyst in synthesis has been widely studied and assessed. Among them, oxovanadium (IV) (vanadyl) compounds were normally treated as pre-catalysts of the corresponding Vanadium (V) species. However, synthetic reactions and pathways that are directly catalyzed by vanadyl species were rarely studied in contrast. Togni in organometallics, 1990 reported the use of camphor-derived vanadyl bis (1,3-diketonato) complexes as Lewis acid catalysts for asymmetric Diels-Alder reactions between Danishefsky dienes and aldehydes. Although a concerted pathway was suggested in the report, the potential amphoteric characteristic of the Vxe2x95x90O unit (i.e., +Vxe2x80x94Oxe2x88x92) which could.be conducive to a stepwise, push-pull type mechanism was not taught. Namely, the (partial) positively charged V in the Vxe2x95x90O unit is a Lewis acid which is sufficient to activate the carbonyl oxygen of an aldehyde. In the meantime, the (partial) negatively charged O in the Vxe2x95x90O unit serves as a Lewis base to activate an enol silane and to permit dissociation of the silyl group concomitantly.
Therefore, the object of the present invention is to provide a process for acyl substitution of an anhydride with an active-hydrogen-containing compound that is devoid of the aforesaid drawbacks of the prior art.
Accordingly, the process for acyl substitution of an anhydride with an active-hydrogen-containing compound includes reacting the anhydride with the active-hydrogen-containing compound in the presence of a vanadyl salt catalyst so as to obtain a high yield of acyl substitution reaction product with high chemoselectivity.
In the process according to this invention, the anhydride is reacted with the active-hydrogen-containing compound in the presence of a vanadyl salt catalyst.
The vanadyl salt catalyst has the following formula:
(VO)X2,
wherein X2 is selected from the group consisting of (OTf)2, SO3-aryl, SO4, (acac)2, (CH3CO2)2, (F)2, (Cl)2, (Br)2, (I)2, Al2O4, (N3)2, SbF6, HPO4, MoO4, (NbO3)2, (NO3)2, C2O4, (ClO4)2, SeO4, Pt (CN)4, TiO3, WO4, and ZrO3.
The active-hydrogen-containing compound has the following formula:
(R1)mxe2x80x94RHnxe2x80x94Cxe2x80x94YH
wherein
Y is selected from the group consisting of O, NH and S;
R is hydrocarbylene group;
each of R1 is independently selected from the group consisting of alkene moiety, ester moiety, lactone moiety, ketone moiety, imide moiety, acetonide moiety, and lactol moiety;
m is 0, 1, 2 or 3;
n is 0, 1, 2, or 3; and
m+n=3
xe2x80x9cOTfxe2x80x9d represents triflate or trifluoromethane sulfonate.
The anhydride typically has the following formula:
(Rxe2x80x2CO)2O
wherein each of Rxe2x80x2 is independently selected from the group consisting of acyclic aliphatic moiety, cyclic aliphatic moiety and aromatic moiety.
A proposed reaction mechanism between an amphoteric vanadyl salt catalyst and an anhydride in the catalytic acylation of an alcohol is illustrated in the following Scheme 1: 
As shown in the above Scheme 1, a protic nucleophile (e.g., the alcohol in the scheme) may add to one of the two alkanoates in the adduct with concomitant elimination of an alkanoic acid to regenerate the vanadyl salt catalyst.
The most preferred vanadyl triflate catalyst can be produced either by reacting vanadyl sulfate with barium triflate or by reacting vanadium oxide and an excess of triflic acid in a suitable solvent.
The process according this invention is useful for the acylation required in the process for preparing sun-screen lotion and surfactant, such as the acylation of sorbitol, glycerin, fatty acid or N,N-dimethyl benzoic acid. The process according to this invention is also useful for the acylation of saccharides and the synthesis of peptides.